Genome-wide association study in alopecia areata implicates both innate and adaptive immunity

Abstract

The genetic basis of alopecia areata, a common autoimmune disease that causes disfiguring hair loss due to the collapse of immune privilege of the hair follicle, is largely unknown. A genome-wide association study has now identified several susceptibility loci for alopecia areata, most of them clustered into eight genomic regions. The spectrum of gene activities affected implies the involvement of both acquired and innate immunity in the condition. Among significant associations are the ULBP genes that encode activating ligands for the natural killer cell receptor NKG2D, which have not been previously linked to autoimmune disease. The genetic basis of alopecia areata, one of the most common human autoimmune diseases, is largely unknown. This study reports a genome-wide association for this trait that implies the involvement of acquired and innate immunity. Among significant associations are the cytomegalovirus UL16-binding protein genes, which encode activating ligands for the natural killer cell receptor, NKG2D, here implicated for the first time in any autoimmune disease. Alopecia areata (AA) is among the most highly prevalent human autoimmune diseases, leading to disfiguring hair loss due to the collapse of immune privilege of the hair follicle and subsequent autoimmune attack1,2. The genetic basis of AA is largely unknown. We undertook a genome-wide association study (GWAS) in a sample of 1,054 cases and 3,278 controls and identified 139 single nucleotide polymorphisms that are significantly associated with AA (P ≤ 5 × 10−7). Here we show an association with genomic regions containing several genes controlling the activation and proliferation of regulatory T cells (Treg cells), cytotoxic T lymphocyte-associated antigen 4 (CTLA4), interleukin (IL)-2/IL-21, IL-2 receptor A (IL-2RA; CD25) and Eos (also known as Ikaros family zinc finger 4; IKZF4), as well as the human leukocyte antigen (HLA) region. We also find association evidence for regions containing genes expressed in the hair follicle itself (PRDX5 and STX17). A region of strong association resides within the ULBP (cytomegalovirus UL16-binding protein) gene cluster on chromosome 6q25.1, encoding activating ligands of the natural killer cell receptor NKG2D that have not previously been implicated in an autoimmune disease. By probing the role of ULBP3 in disease pathogenesis, we also show that its expression in lesional scalp from patients with AA is markedly upregulated in the hair follicle dermal sheath during active disease. This study provides evidence for the involvement of both innate and acquired immunity in the pathogenesis of AA. We have defined the genetic underpinnings of AA, placing it within the context of shared pathways among autoimmune diseases, and implicating a novel disease mechanism, the upregulation of ULBP ligands, in triggering autoimmunity.